With the rapid development of digitalization of information, digitalization in image processing is increasingly required. In digital cameras in particular, solid-state image pickup devices, such as Charge Coupled Devices (CCD) and Complementary Metal Oxide Semiconductor (CMOS) sensors, have been used instead of film.
In image pickup apparatuses including CCDs or CMOS sensors, an image of an object is optically taken by an optical system and is extracted by an image pickup device in a form of an electric signal. Such an image pickup apparatus may be used for reading close-up still images such as a barcode, an iris, or a character.
In lens systems used for optical imaging, depth of field is an important characteristic of the lens system. Depth of field is, without limitation, a portion of an imaged region (field) that appears sharp in an image. Since a lens system has a characteristic that a focus amount increases as an object distance decreases, depth of field extension is often useful. In barcode reading, for example, the depth of field can be extended by decreasing an aperture of the optical system. Unfortunately, when the aperture of the optical system is decreased, the amount of light decreases. As a result, a shutter speed required for imaging decreases, and a risk of motion blurring may increase.
Alternatively, automatic focusing can be used for depth of field extension. However, a driving section for automatic focusing is necessary, and this increases a cost and reduces durability and reliability of the image pickup apparatuses. Accordingly, a high-performance lens, such as a multifocal lens for extending the depth of field or a lens for extending the depth of field with a phase modulation element, is sometimes used. However, optical performance of the lens system may be reduced by an influence of reflection and diffraction by an edge in the high-performance lens having a discontinuous shape caused by variations in manufacturing of the lens system. Further, since a point-spread-function of the high-performance lens may be rotationally asymmetric, a size of a restoration filter of the high-performance lens increases, and a false image may be formed when restoration is insufficient.
Therefore, there is a need for an optical system for extending depth of field that uses a filter of a smaller size for image restoration while suppressing sensitivity to variations in manufacturing of the optical system.